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DIY: Do It Yourself Post here to share or improve your wrench turning skills! All BMW E46 DIY tips, tales, and projects discussed inside. Learn to work on your car and know the right BMW parts you will need!

Careful where you source your regulator. I got a $40-50 dollar Bosch OEM unit from Pelican and my headlights are pulsing. I now need to replace the regulator again and will be going with a $120 unit from BavAuto. I susspect the Pelican one was a refurb.

I just replaced my the voltage regulator Bosch 120A on my 02 325i.
Some information on testing the alternator diode pack (Rectifier) that might be useful to someone diagnosing the alternator.

1.> Once you remove the cover from the alternator, you will see six soldered terminals around the back of the alternator. These are obviously the stator windings ends.
2.> To test if your diodes are ok, you need to put your DMM (multimeter) in the diode test mode ( -|>|- ) symbol.
3.> Touch one DMM lead to the soldered winding end and the other to the long bolt (The alternator output, B1+) terminal. This will either give you '1' or some number (I got 436) that is the voltage drop across the diode in mV. Reverse the leads, the readings should be reverse of what you got earlier. i.e. if you got a '1' before, you will get some number now and vice a versa.
4.> Repeat this test with all the six exposed soldered ends. In each case the the reading should reverse. if you get same reading in both directions, You have a failed diode and your alternator will be putting out AC voltage.
5.> Now repeat this test with the smaller bolt coming out of the alternator.
6.> Again repeat this test between the alternator body (ground) and the winding ends. You should get the same results. Same reading in both directions from step 4,5,6 indicating a failed diode inside.

DO NOT DO THIS WITH THE CAR RUNNING. DISCONNECT THE BATTERY +ve while doing this.

This will tell you if you can get away with just changing the regulator or not.

Please add/correct if anybody has any further inputs on this.

Also, the screws holding the regulator in place are M4x20 with 0.7 pitch and can be found at Ace Hardware.

Careful where you source your regulator. I got a $40-50 dollar Bosch OEM unit from Pelican and my headlights are pulsing. I now need to replace the regulator again and will be going with a $120 unit from BavAuto. I susspect the Pelican one was a refurb.

I used a small mirror to "see" the nuts/screw I had to remove in order to get the plastic cover off the back of the alternator......

If you were facing the back of the Bosch 120-amp alternator, how does one remove the VR cover fastener in what would be the 3 o'clock position? This being the fastener with the round plastic cover you pop off with a finger. It's the one labeled "1" in the first photo in the OP.

So I did the DIY, and everything started going as advertised. About ten minutes into my drive after having installed the new voltage regulator (did it without removing the alternator), everything was going well. Then all of a sudden I check my battery and it's back down to 12.1-12.2V after having been 13.8-14.0. I stop the car and upon re-starting it.. The dreaded red battery light is back. I installed a new Duralast 49-DL exactly a week ago.

Sounds like the first thing to do would be to check all your wires are secure and there isn't any corrosion. Especially the grounds.

But it sounds like your new regulator is a dud. How much did you pay for it? If you got a good deal you probably got a refurbished one. I replaced mine the first time with a $50 one from Pelican parts and had oscillating headlights. Turns out it was a refurb. Second time I paid full price for a new one from Bav Auto. Works like a charm.

The good news is that with a regulator in your hand and the experience of doing this once already, you can replace the regulator without removing the whole alternator (though it does require some dexterity to get your hands around the back of the alternator.

Sounds like the first thing to do would be to check all your wires are secure and there isn't any corrosion. Especially the grounds.

But it sounds like your new regulator is a dud. How much did you pay for it? If you got a good deal you probably got a refurbished one. I replaced mine the first time with a $40 one from Pelican parts and had oscillating headlights. Turns out it was a refurb. Second time I paid full price for a new one from bad Auto. Works like a charm.

The good news is that with a regulator in your hand and the experience of doing this once already, you can replace the regulator without removing the whole alternator (though it does require some dexterity to get your hands around the back of the alternator.

I bought the one from Pelican for $50, but haven't noticed pulsating headlights or anything of that nature. Everything was going perfectly and then all of a sudden the voltage dropped.

I emailed them and they said to return in for a replacement. That was a year after I had originally ordered it. Since it was just headlights pulsing I actually drove around for months before I replaced it again. They offered replacement and since I wanted to go with a BavAuto new one I never took them up on the offer.

I emailed them and they said to return in for a replacement. That was a year after I had originally ordered it. Since it was just headlights pulsing I actually drove around for months before I replaced it again. They offered replacement and since I wanted to go with a BavAuto new one I never took them up on the offer.

So were you able to get your money refunded? Was it ever confirmed that the voltage regulators that they sell are actually refurbs?

As of right now, I'm just hoping it's all user error and myself not tightening everything fully. I would suspect that, too, actually, since I was in a bit of a rush.. I guess being an 18 year old I can get pretty anxious and whatnot. Going to go back in soon and check everything.

So I got up early today & pulled everything out again, checking all of my connections.. Turns out one of the nuts that were connected to the positive connector was terribly loose..

It goes like this:

15mm hex nut | positive connector | 17mm hex nut

The 17mm hex nut was snug, as I suspected (because I clearly remembered tightening it well enough), but upon removing that and the positive connector which was in between, the 15mm hex nut spun freely. I was able to screw it out with my fingers. That was the likely culprit, so I just took everything else out and re-installed everything back, making sure they were all tight and secure. This DIY guide didn't have any torque values (I'd image it'd be an absolute pain to get a torque wrench in there, anyways) so I just tightened everything to a point where they wouldn't budge and it'd take some effort to break the seize, but not too much (I've made that mistake in the past already...). Upon installing everything else back, putting the PS reservoir in place, and the air box back into position, I fired the car up with the voltage showing via OBC, and I was delighted to see it back to normal!

I haven't driven it yet, though, but I will for a good while (and near my neighborhood!) to be sure. When the car is off, it gives me a reading of 12.2-12.3V, when it's idle it gives me 13.5-13.6V (when cold, I haven't checked fully warmed up yet) and when I drive it and go to around 3K RPM's it should be around 13.8-14.1V. I did rev the engine in neutral to about 3K and it gave me 13.7-13.8V (again, cold, as I'm not sure whether or not it'd be different), but I'd suspect it would be different when in gear? How does that sound?

Thanks to all who posted on this thread I just replaced my vr without removing the alternator. You do need patience (surgical hands do help since it is a bit tight) and a mirror to do this but it is doable. You just gotta remember how everything goes and make sure everything is tight.

My main issue is that my alt has the vr with the oval connector instead of the rectangular connector as listed so after buying the rectangular one on Pelican I had to return it. Found the part on Autozone thanks to another thread posted here. I was surprised my local dealer didn't have the oval connector (or it wasn't showing in their systems as the part for my car so they didn't even think to look - I believe this part shows for the Z4). The bushes on the old vr were pretty worn as you can see below.